Investigate flighting new virtual environments images in our hosted agent pool

[benbp]

Our devops agent pool is managed by another team, part of which includes automatic VM version upgrades from the VM images specified at https://github.com/actions/virtual-environments/. Recently we had an issue where the latest VM image included a git cli version upgrade to 2.37.0 which broke our usage of sparse checkout (switching to cone mode by default. See #3606 for more details).

Ideally, we would have been able to catch this issue if we were able to preview new VM image updates coming to our agents several days in advance, before they roll out to the entire VM agent pool. I'm not sure yet what a solution here would look like, whether it's a feature we can ask for from the agent pool team, or whether we would need to build the VM images ourselves and attempt to run a sampling of pipelines on a separate hosted pool.